Pointer for Nitpicker with optional hover-sensitive shape support

Per default the standard "big mouse" pointer is rendered on screen.

Additionally, 'pointer' supports to render "pointer shapes" when hovering
Nitpicker sessions for which a shape was reported. This feature must be
enabled by setting the 'config' attribute 'shapes' to 'yes'. When enabled, the
'pointer' component announces a 'Report' service for custom pointer shapes
and requests ROM sessions for Nitpicker's 'hover' and 'xray' reports. The
mapping between hovered Nitpicker sessions and applications can be achieved
with report session label rewriting:

! + start shape-arrow | ram: 2M
!   + binary test-pointer
!   + config | shape: arrow
!   + route
!     + service PD | + parent
!     + service ROM | + parent
!     + service CPU | + parent
!     + service LOG | + parent
!     + service Report | label: shape
!       + child pointer | label: test-label-arrow -> shape

In the example above, which is from 'pointer.run', the 'shape-arrow' component
reports an arrow shape with the label "shape". By rewriting the label of the
report, the shape will be drawn for the 'test-label-arrow' component when
its Nitpicker view is hovered.

Technically, the 'pointer' component compares the hovered label, which is
'test-label-arrow -> testnit' in this case, with the shape report label after
stripping the last element of each label, so the remaining label prefix
'test-label-arrow' is the actual match criteria.

When both the 'shapes' attribute and 'verbose' attribute are set to 'yes', the
pointer component prints the labels of hovered GUI sessions and the received
shape reports to the log.

The most common use cases for pointer shapes are VirtualBox, which reports
the guest-pointer shapes if Guest Additions are installed, and Qt applications.

The pointer can be scaled using a whole-number factor specified in the 'scale'
attribute. For using different factors for vertical and horizontal scaling,
the attributes 'scale_x' and 'scale_y' can be used.

Default pointer shape
---------------------

Whenever no client-provided shape is available, the pointer falls back
to a default shape. This shape can be customized using a 'shape' node
named 'default'. The pixels are provided in a character-encoded format as
illustrated in the following example.

: + shape default | x: 0 | y: 0
:   + color . | rgb: #000000
:   + color ; | rgb: #000000 | alpha: 0.5
:   + color , | rgb: #000000 | alpha: 0.2
:   + color # | rgb: #ffffff
:   + pixels
:     : ##
:     : #.#
:     : #..#
:     : #...#
:     : #....#
:     : #.....#
:     : #......#
:     : #.......#
:     : #........#
:     : #.....#####
:     : #..#..#;;;;
:     : #.#;#..#:::
:     : ##;:#..#
:     : #;: ;#..#
:     : ;:  :#..#
:     : :    ;###
:     :      :;;;
:     :       :::

Each color is represented by a distinct ASCII character. A 'color' node
assigns an RGB color value and an optional alpha value to a character.
The pixels of the shape are defined as quoted content of the 'pixels' sub
node, using the defined characters. Any character with no color assignment
is assumed to be transparent.

The 'x' and 'y' attributes of the 'shape' define the hot-spot position within
the shape. The shape is placed on screen such that this spot matches the
pointer position.
